1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium such as a photosensitive member.
2. Related Background Art
The printers for electrophotographic image reproduction by turning on and off a laser beam according to image signals can be classified into two categories with respect to the correlation between the reproduced image and the on-off state of the laser beam.
The first category is image scan process printers, in which the laser beam is turned on at the positions of black pixels where the image is reproduced and is turned off at the positions of white pixels where the image is not reproduced. The second category is background scan process printers, in which the laser beam is turned off at the positions corresponding to black pixels and turned on at the positions of white pixels.
FIG. 9 illustrates an image forming unit of a laser beam printer, in which a laser unit 91 turns on and off a laser beam in response to image signals supplied thereto. The laser beam emitted therefrom is guided through a collimating lens 90, then reflected by a polygon mirror 92 rotated at a constant speed, and is focused on a photosensitive member 92 after an optical path correction by a correction optical system 93. Due to the illustrated rotation of the polygon mirror 92, each reflecting face of the mirror causes the laser beam to perform one scan of the photosensitive member 92 in a direction Hs. A two-dimensional scanning is achieved by the rotation of the photosensitive member 92 at a constant speed, thereby achieving image reproduction.
A part of the scanning beam is reflected by a mirror 94 and is subjected to photoelectric conversion in a photosensor 95 to generate a horizontal synchronization signal or a beam detection signal BD.
The images printed by such laser beam printer are predominantly document images including characters and symbols, in which the number of white pixels (an area) is generally much larger than that of black pixels. The cumulative turn-on time of the laser beam is therefore smaller in the image scan process in which the laser beam is turned on at the positions of black pixels, so that the image scan process is preferable to the background scan process in the case of using a semiconductor laser which is concerned about the service life of the laser source.
On the other hand, such image scan process often shows difficulty in the reproduction of so-called solid black image consisting of consecutive black pixels, since the non-uniformity in reflecting faces of the polygon mirror and the mechanical vibration result in a fluctuation in the pitch of scanning lines, thus giving rise to striped patterns in the image in the scanning direction.
On the other hand, the background scan process can reproduce uniform black image since the laser beam is not turned on at the positions of black pixels.
Also in the reproduction of a fine line of a width of about one pixel, the image scan process and the background scan process exhibit different behaviors. This is due to the fact that the laser beam spot focused on the photosensitive member has an approximately Gaussian two-dimensional energy distribution, and the development characteristic for visualization of a latent image formed on the photosensitive member by such laser beam spot. FIGS. 10A and 10B illustrate such fine line reproduction with the image scan process. In the reproduction of a black line of a width of one pixel as shown in FIG. 10A, the laser beam is turned off at the pixels represented by white circles, and is turned on at the pixels represented by black circles. The width l.sub.1 of the reproduced image becomes wider because of the trailing energy distribution of the laser beam spot on the photosensitive member explained above and the electrophotographic development characteristics. On the other hand, a white line of a width of one scanning line becomes thinner due to same reason, as shown in FIG. 10B. Thus small characters and symbols become illegible. Such a tendency becomes more marked when the beam spot diameter and the development conditions are so selected that the neighboring scan lines mutually overlap in order to prevent the aforementioned unevenness in the solid black area caused by the errors in the scanning optical system and particularly a white line of the width of one scanning line becomes buried in the black areas and is not reproduced.
FIGS. 11A and 11B illustrate the case of the background scan process. Contrary to the image scan process, a black line as shown in FIG. 11A becomes thinner and a white line as shown in FIG. 11B becomes thicker. Consequently the letters and symbols are reproduced thinner, and, a thin line of a width of one scanning line cannot be reproduced at all depending on the beam spot diameter and the developing conditions because of the reasons explained above.
As explained above, the image scan process and the background scan process respectively have advantages and drawbacks.
On the other hand, the present applicant already proposed, for improving the reproducibility of intermediate tone images with a laser beam printer, a process of image reproduction by comparing an image signal having gradations with a periodical signal such as a triangular signal synchronized with the image signal, and generating a signal of a pulse width corresponding to the tone of the image signal thereby effecting pulse width modulation of the laser beam. In this process, the tonal rendition is not provided in the dot density itself but is obtained by varying the ratio of black area and white area per unit area, utilizing the human visual characteristics.
This process can be utilized for correcting the aforementioned pixel thinning or thickening, arising from the above-explained scanning process in the laser beam printer.
FIG. 12 shows the basic structure of the process. A digital image signal VIDEO with intermediate tone is synchronized with an image clock signal .phi..sub.T, by a latch 1201, is then converted into a voltage corresponding to the image density level by a D/A converter 1202 and a resistor 1203, and is supplied to an input terminal of a comparator 1204.
On the other hand, a triangular wave synchronized with the image clock signal .phi..sub.T is generated by a triangular wave generator 1205 and is supplied to the other input, terminal of the comparator 1204, which compares the triangular wave with the image signal converted into the voltage. Thus a binary signal of a pulse width corresponding to the level of the image signal can be obtained and is utilized to drive the laser, thereby enabling tonal image reproduction.
This process can be utilized for coping with the pixel thinning or thickening arising from the aforementioned scanning processes of the laser beam printer. More specifically, it is possible to examine the relation, in signal level, between a target pixel and neighboring pixels, and to change the modulation characteristics of the pulse width if said target pixel is identified as a part of a thin line. For example, in the image scan process in which a black thin line tends to be reproduced thicker, the pulse width for a pixel is made narrower if the pixel is identified as a part of a black thin line.
However, though this process can be effective for a thin line perpendicular to the beam scanning direction, it cannot be effective for a thin line parallel to the beam scanning direction.
Besides the present applicant already proposed various methods, in the U.S. Pat. Nos. 4,387,983, 4,517,579 and 4,476,474, for preventing image quality deterioration such as image thinning or thickening, but a satisfactory effect has not been attained.
Furthermore the copending U.S. Ser. No. 898,096 filed on Aug. 20, 1986 proposes a method of faithfully reproducing an intermediate tone image by varying the pulse width for turning on a beam as well as the beam intensity, but such method is still incapable of satisfactorily preventing the image quality deterioration as explained above.
Furthermore the copending U.S. Ser. No. 091,773 filed on Sept. 1, 1987 proposes to control the beam intensity by identifying the image characteristic, but a further improvement has been desired.